Prosthetic limbs have come a long way since the days of simple wooden “peg legs”. Today, amputee men running on a prosthetic leg can beat race times of the best unimpaired women runners. It is believed that new advances in prosthetic limbs (such as those embodied in the present invention) will soon lead to amputees being able to out-perform the best unimpaired athletes of the same sex in sports such as running. It is an object of the present invention to advance the state of prosthetic limbs to a new level, providing increased athletic performance, increased control, and reduced body strain. It is a further object of the present invention to provide essential elements needed for making prosthetic limbs that more accurately mimic the mechanical behavior of healthy human limbs.
Description of Normal, Level-Ground Walking:
In order to establish terminology used in this document, the basic walking progression from heel strike to toe off is first explained. There are three distinct phases to a walking stance-period as depicted in FIG. 1 with heel-toe sequence 1 through 7.
Saggital Plane Knee Phases
                1. Beginning with heel strike, the stance knee begins to flex slightly (Sequence 1-3). This flexion allows for shock absorption upon impact as well as keeping the body's center of gravity at a more constant vertical level throughout stance.        2. After maximum flexion is reached in the stance knee, the joint begins to extend again, until full extension is reached (Sequence 3-5).        3. During late stance, the knee of the supporting leg begins to flex again in preparation for the swing phase (Sequence 5-7). This is referred to in the literature as ‘knee break’. At this time, the adjacent foot strikes the ground and the body is in “double support mode” (that is to say, both legs are supporting body weight).Saggital Plane Ankle Phases        1. Beginning with heel strike, the ankle undergoes a controlled plantar-flexion phase where the foot rotates towards the ground until the forefoot makes contact (Sequence 1-2).        2. After controlled plantar-flexion, the ankle undergoes a controlled dorsi-flexion phase where the tibia rotates forwardly while the foot remains in contact with the ground (Sequence 2-5).        3. During late stance, the ankle undergoes a powered plantar-flexion phase where the forefoot presses against the ground raising the heel from the ground (Sequence 5-7). This final phase of walking delivers a maximal level of mechanical power to the walking step to slow the fall of the body prior to heel strike of the adjacent, forwardly positioned leg.        
The development of artificial leg systems that exhibit natural knee and ankle movements has been a long standing goal for designers of legged robots, prostheses and orthoses. In recent years, significant progress has been made in this area. The current state-of-the-art in prosthetic knee technology, the Otto Bock C-Leg, enables amputees to walk with early stance knee flexion and extension, and the state-of-the-art in ankle-foot systems (such as the Össur Flex-Foot) allow for ankle controlled plantar-flexion and dorsi-flexion. Although these systems restore a high level of functionality to leg amputees, they nonetheless fail to restore normal levels of ankle powered plantar-flexion, a movement considered important not only for biological realism but also for walking economy. In FIG. 2, ankle power data are shown for ten normal subjects walking at four walking speeds from slow (½ m/sec) to fast (1.8 m/sec). As walking speed increases, both positive mechanical work and peak mechanical power output increase dramatically. Many ankle-foot systems, most notably the Flex-Foot, employ springs that store and release energy during each walking step. Although some power plantar-flexion is possible with these elastic systems, normal biological levels are not possible. In addition to power limitations, the flex-foot also does not change stiffness in response to disturbances. The human ankle-foot system has been observed to change stiffness in response to forward speed variation and ground irregularities. In FIG. 3, data are shown for a normal subject walking at three speeds, showing that as speed increases ankle stiffness during controlled plantar-flexion increases.
Artificial legs with a mechanical impedance that can be modeled as a spring in parallel with a damper are known in the art. Some prostheses with non-linear spring rates or variable damping rates are also known in the art. Unfortunately, any simple linear or non-linear spring action cannot adequately mimic a natural limb that puts out positive power during part of the gait cycle. A simple non-linear spring function is monotonic, and the force vs. displacement function is the same while loading the spring as while unloading the spring. It is an object of the present invention to provide actively electronically controlled prosthetic limbs which improve significantly on the performance of artificial legs known in the art, and which require minimal power from batteries and the like. It is a further object of the present invention to provide advanced electronically-controlled artificial legs which still function reasonably well should the active control function fail (for instance due to power to the electronics of the limb being lost). Still further, it is an object of the present invention to provide artificial legs capable of delivering power at places in the gait cycle where a normal biological ankle delivers power. And finally, it is an object of the present invention to provide prosthetic legs with a controlled mechanical impedance and the ability to deliver power, while minimizing the inertial moment of the limb about the point where it attaches to the residual biological limb.
During use, biological limbs can be modeled as a variable spring-rate spring in parallel with a variable damping-rate damper in parallel with a variable-power-output forcing function (as shown in FIG. 4a). In some activities, natural human limbs act mostly as spring-damper combinations. One example of such an activity is a slow walk. When walking slowly, a person's lower legs (foot and ankle system) act mostly as a system of springs and dampers. As walking speed increases, the energy-per-step put out by the muscles in the lower leg increases. This is supported by the data in FIG. 2.
Muscle tissue can be controlled through nerve impulses to provide variable spring rate, variable damping rate, and variable forcing function. It is an objective of the present invention to better emulate the wide range of controllability of damping rate, spring rate, and forcing function provided by human muscles, and in some cases to provide combination of these functions which are outside the range of natural muscles.